1. Field of the Invention
The present invention relates generally to optical scanning systems, and particularly to a two-mirror reflective scanning system in which the motion of the two mirrors are coupled to provide a wide field of view to an optical sensor.
2. Description of Related Art
Many kinds of optical scanning systems have been developed in recent years. These systems are used for photographing and measuring light in both airborne and ground based systems. For example, ground based scanners may be used in systems such as telescopes, radiometers, motion picture camera systems and laser scanners. Optical scanners are also found in a variety of airborne applications, including, for example, aircraft, spacecraft and satellites. Both ground based and airborne scanners may be used for observation of the Earth, objects on the Earth or objects in Space.
In general, the light output of an optical scanner is directed onto a light sensor. For example, the sensor may be a photographic film, an electronic photodetector or the human eye. The spectrum of light viewed by optical scanners may be a narrow bandwidth or wide bandwidth and this spectrum may range from infrared through ultraviolet.
A basic requirement of all scanning systems is the ability to scan, i.e. to direct light from a range of angular positions to a sensor with a limited field of view. Scanning requires motion of one or more optical devices to sweep the light across a fixed sensor. Unfortunately, a number of changes in the light image are often introduced by this motion of the scanner's optical devices during the scanning process. Additional changes may be introduced when the incoming light is reflected or refracted by the optical devices in the scanner. These alterations are generally undesirable. They include distortion, image rotation, polarization and vignetting. Numerous systems have been developed in an effort to avoid or compensate for these alterations in the image. A further discussion of scanning systems may be found in J. Marshall, "Scanning Devices and Systems" in Applied Optics and Optical Engineering, Vol. 6, pp. 204-262 (1980), and also in W. Wolfe, "Optical--Mechanical Scanning Techniques and Devices" in The Infrared Handbook, Office of Naval Research, chapter 10, (1978).
It will be appreciated that undesirable alterations in the optical image are increased as the field of view is increased. For example, scanning over a wide field of view will usually result in considerable image rotation, image distortion and vignetting. Another factor is the type of optical device utilized. Optical scanning systems may use reflective or refractive optical components or a combination of these two. However, refractive systems of large diameter generally are more expensive and may introduce greater distortion and polarization of the image than reflective systems.
In sum, it would be desirable to have a optical scanning system which can scan over a wide field of view, for example, 60 degrees, without significant rotation of the image, without vignetting and without image distortion. It would also be desirable to have such an optical system utilize inexpensive reflective components rather than refractive components.